Using Membrane Sets Incorporated into a Crossflow Electrofiltration/Electrodialysis Treatment Module to Treat CMP Wastewater and Simultaneously Generate Electrolytic Ionized Water

by Yang, Tsung-Yin

Abstract (Summary)

In this work, membrane set(s) had been incorporated into different crossflow electrofiltration (CEF) /electrodialysis (ED) treatment modules for treating various CMP wastewaters and simultaneously generating two streams of electrolytic ionized water (EIW). In general, CMP wastewaters have high alkalinity, turbidity, total solids content and silica content. In this investigation, CMP wastewaters were obtained from two wafer fabs in Taiwan and characterized by various standard methods. Then they were treated by the aforementioned treatment modules. Experiments were carried out based on the fractional factorial design and the L8 orthogonal arrays of the Taguchi method. Experimental factors such as electric field strength, transmembrane pressure for CEF, etc. were used to investigate their effects on the permeate qualities (i.e., oxidation-reduction potential, pH, etc.). According to the results of analysis of normal probability plots, analysis of variance (ANOVA) and regular analysis, the electric field strength was presumed to be a very significant parameter. Experimental results showed that filtrate flux increased with the increasing applied electric field strength. The permeate has a turbidity of below 1 NTU, TOC of below 3 mg/L, and TDS of below 250 mg/L under various operating conditions. Other permeate qualities were 15~22 mg/L of K, 53~68 mg/L of silica, 2~4 mg/L of NH4+ and 134~680 £gS/cm of electrical conductivity. But the values of electrical conductivity, pH, and oxidation-reduction potential (ORP) varied substantially for the anolyte EIW and catholyte EIW. Using these novel treatment modules, the optimal ORP and pH values of the anolyte EIW were 211.8 mV, 4.52 and 214.1 mV, 4.83, respectively, for single- and multi-membrane sets. The optimal ORP and pH values of the catholyte EIW were -165.0 mV, 11.21 and -172.0 mV, 10.81, respectively, for single- and multi-membrane sets. It is clear that permeate obtained in this study is suitable for high-level recycling. To further upgrade the water quality of permeate obtained above, a reverse osmosis (RO) unit was added to the treatment system. The water quality of silica for post-RO permeate were decreased from 53.7 to 0.98 mg/L for the anolyte EIW and from 68.05 to 1.32 mg/L for the catholyte EIW. The removal rates of Na and K by the RO unit were not significant. In addition, other unique properties of EIW (e.g., pH, ORP, and cluster size of water molecules) remained almost the same in post-RO permeate. The total recovery rate of the treated water could be above 85%. Therefore, the treated water at this stage could be reused as the cleaning media for the wafer surfaces or reused for the DI water production apparatus.